Device for varying the damping of a tuned oscillatory circuit without affecting the tuning



uly 8, 19 1- 'r. A. sPooR ETAL 2,248,470

DEVICE FOR VARYING THE DAMPING OF A TUNED QSCILLATORY CIRCUIT WITHOUTAFFECTING THE TUNING Filed April 27, 1938 INVENTORS' T. A SPOOR H- RIN/ABY EA. V Y REN ATTbRNEY Patented July 8, 1941 DEVICE FOR VARYING THEDAMPING OF A TUNED OSCILLATORY CIRCUIT WITHOUT AFFECTING THE TUNINGTheodorus Antonius Spoor, Herre Rinia, and

Ewoud Adriaan van Yzeren,

Eindhoven,

Netherlands, assignors, by mesne assignments, to Radio Corporation ofAmerica, New York, N. Y., a corporation of Delaware Application April27, 1938, Serial No. 204,596 In Germany June 7, 1937 6 Claims.

This invention relates to a device for varying the damping of a tunedoscillatory circuit without the tuning being at the same time afi'ectedby this damping variation.

With band-pass filters, whose band width is controlled by varying themutual coupling of the band-pass filter circuits the drawback oftenoccurs that with a great width of the frequency band transmitted a verydeep dale ensues in the middle of the resonance curve, due to which thelowest modulation frequencies are weakened with respect to the higherfrequencies so that the reproduction is distorted. This drawback may beobviated by increasing the damping of the bandpass filter circuits withincrease in mutual coupling of these circuits. However, this dampingincrease of the band-pass filter circuits must take place withoutvarying the tuning of the circuits, since such variation would involve anew distortion of the reproduction or would make it necessary to tunethe receiver anew after any variation of the band-width.

With a well known solution of this problem variable resistances areinserted in the tuned oscillatorycircuits of which the band-pass filterconsists, the value of said resistance being altered in accordance withthe width of the transmitted frequency band by mechanically coupling thead- J'usting member, by means of which the bandwidth is adjusted, to theadjusting member of the variable resistances. However, this solution hasvarious drawbacks, the device being expensive and bulky, whilst uponadjusting the variable resistances undesirable crackling cannot beavoided.

The present invention provides another solution of the problem ofvarying the damping of a tuned oscillatory circuit without at the sametime affecting the tuning and whilst avoiding the drawbacks concomitantwith the known solution referred to above.-

According to the invention the damping of a tuned oscillatory circuit isvaried by shifting and/or turning the inductance coil of the oscillatorycircuit relatively to a damping circuit whose resistance is large withrespect to the inductance and which is inductively coupled to the coil.By a damping circuit is to be understood in this case any conductivebody in which currents causing damping of the oscillatory circuit can beinduced by the field of the inductance coil of the oscillatory circuit.

The damping circuit may consist, for instance of a coil which is coupledto the inductance coil of the oscillatory circuit, and is wound fromresistance wire, whilst its ends are directly interconnected. Thisdamping coil is either rotatably or movably arranged relatively to theinductance coupling between the damping circuit and the oscillatorycircuit.

In another form of construction the inductance coil of the oscillatorycircuit is movably arranged on a massive core partly consisting of amaterial having a high specific resistance (for instance,

iron) and partly of a material having a smaller specific resistance(such as copper), the two parts of the core having the same diameter. Ifthe inductance coil is arranged on the core portion consisting ofmaterial having a high specific resistance, great losses will occur inthe core so that a material damping of the oscillatory circuit will beobtained. If, on the contrary, the inductance coil is provided on thecore portion consisting of material having a low specific resistance thedamping of the oscillatory circuit is small. The small variation of theself induction of the coil caused by the available core is substantiallyequal for the two core portions, since the two parts have the samediameter, so that the displacement of the inductance coil over the coreinvolves only a very slight variation of the tuning of the oscillatorycircuit which may be further reduced, if desired, by giving the two coreparts a slightly different diameter. The losses incurred when theinductance coil is provided on the core portion made from materialhaving a low specific resistance (for instance copper) may be furtherreduced by lodging a ring of copper or similar material in the screeningbox containing the coil. As an alternative, the bottom of the screeningcase may be used for this purpose, if desired.

In another very simple construction, whose effect corresponds to that ofthe above construction, a segment consisting partly of material having ahigh specific resistance (for instance iron) and partly of a materialhaving a low specific resistance (for instance copper) is arranged infront of the head of the movably arranged inductance coil of theoscillatory circuit, so that upon rotation of the inductance coil thelatter is more closely coupled either to the part consisting of materialhaving a high specific resistance or to the part-made from materialhaving a low specific resistance. The same efiect can also be obtainedwhen two vertical plates consisting of materials having a high and a lowspecific resistance respectively are arranged inside the rotatablyarranged inductance coil of the oscillatory circuit.

In the two last-mentioned forms of construction the portion of thedamping circuit made from material having a low specific resistancemight be dispensed with if the thickness of the part made from materialhaving a high specific resistance be very small, i. e. small relativelyto.

the depth over which the field of the inductance coil can penetrate intothe material. On this recognition is based another form of constructionof the invention which resides in the arrangement of a very thin platefrom material having a high specific resistance such as constantan inthe vicinity of the movably arranged inductance coil of the oscillatorycircuit. Upon rotation of the inductance coil the plate is to a greateror less degree brought into the field of the coil due to which thedamping of the oscillatory circuit is varied.

Although the invention may be used in any case where damping variationof an oscillatory circuit without simultaneously affecting the tuning isdesirable, the invention is of particular importance for band-passfilters with variable band-width in which the latter is varied by mutualdisplacement or rotation of the inductance coils of the band-pass filtercircuits, since in such band-pass filters the mutual rotation ordisplacement of the inductance coils may at the same time be used forcoupling in any of the above ways and to a greater or less degree atleast one of these to a damping circuit. In this manner the drawbackreferred to in the preamble, viz. that a very deep dale occurs in themiddle of the resistance curve, with a great width of the transmittedfrequency band, can be obviated in an extremely simple manner.

The invention will be more clearly understood by reference to theaccompanying drawing wherein Figs. 1 and 2 disclose in diagrammaticfashion two embodiments thereof.

Fig. 1 shows two coils L1 and L2 forming part of the two oscillatorycircuits of a band-pass filter whose bandwidth can be altered byshifting the coil L2 relatively to the coil L1. The two coils are lodgedin a screening box B. The movable coil L2 passes over a massive core Kconsisting partly of iron (Fe) and partly of copper (Cu). With theweakest coupling the coil L2 occupies the extreme right position andthis coil is then coupled almost exclusively to the copper part of thecore K. In this case the losses induced in the coil are very small andare further reduced, moreover, by a copper ring R lodged in thescreening box B. If the coupling is made closer by shifting the coil L2to the left, this coil is steadily coupled more closely to the ironportion of the core K which involves a material increase of the losses.Instead of iron and copper it is also possible to use other materials,for instance two kinds of iron having different specific resistances.Any slight self induction variations occurring upon shifting the coil L2may be reduced by giving the two parts of the core a slightly differentdiameter.

Fig. 2 shows two coils L1 and L2 forming part of the two oscillatorycircuits of a band-pass filter, whose band-width can be varied byrotation of the coil L2 about the axis A. A segment S consisting partlyof iron (Fe) and partly of copper (Cu) is arranged opposite the coil L2.In the illustrated position of the coil L2 the coupling is at itsmaximum and the coil L2 is mainly coupled to the iron part of thesegment S thus incurring great losses. Upon loosening the coupling byrotation of the coil L2 in a clockwise direction this coil is steadilycoupled more closely to the copper part of the segment S due to whichthe losses decrease. Instead of the segment shown in Fig. 2 it is alsopossible to use a very thin plate from constantan or similar materialwhich is arranged in such a manner that in the position illustrated itis closely coupled to the coil L2, the coupling upon rotation of thecoil L2 decreasing continuously in the manner set out above.

We claim:

1. A device for varying the damping of a tuned oscillatory circuitwithout substantially affecting the tuning thereof, characterized inthat the inductance coil of the oscillatory circuit is movably arrangedon a massive core partly consisting of a material having a high specificresistance and partly of a material having a low specific resistance.

2. A device for varying the damping of a tuned oscillatory circuitwithout substantially affecting the tuning thereof, characterized inthat the inductance coil of the oscillatory circuit is rotatablyarranged relatively to a segment partly consisting of a material havinga high specific resistance and partly of a material having a lowspecific resistance.

3. A device for varying the damping of a tuned oscillatory circuitwithout substantially affecting the tuning thereof, comprising astationary metallic member coaxially arranged with respect to andinductively coupled to the coil of said oscillatory circuit, said memberconsisting of portions which have different specific resistances,whereby axial movement of the coil to selectively cooperate with one orthe other of said different portions of the core varies the damping ofthe circuit Without substantially affecting its tuning.

4. A device for varying the damping of a tuned oscillatory circuitwithout substantially affecting the tuning thereof, comprising astationary core member coaxially arranged with respect to the coil ofsaid oscillatory circuit and upon which said coil is mounted forrelative movement, said core consisting of portions which have differentspecific resistances, whereby adjustment of the coil to selectivelycooperate with one or the other of said different portions of the corevaries the damping of the circuit without substantially affecting itstuning.

5. A device according to the invention defined in claim 4, wherein thecore portions are iron and copper.

6. An adjustable band-pass filter comprising a pair of oscillatorycircuits, the coupling therebetween for varying the band width beingchanged by altering the relative positions of the circuit coils, andmeans cooperating with one of the coils for varying the damping thereof,said means comprising a member coaxially arranged with respect to saidone coil and having portions which have different specific resistances,the arrangement being such that with variation of the coupling toproduce close coupling the movable coil encircles the portion having thehigher specific resistance and is affected only by said portion causingthe circuit to be highly damped, and with variation of the coupling toproduce loose cou pling the movable coil encircles the portion havingthe lower specific resistance and is affected only by said portioncausing the circuit to be slightly damped.

TI-IEODORUS ANTONIUS SPOOR. HERRE RINIA. EWOUD ADRIAAN VAN YZEREN.

